Since the discovery of oxide superconducting materials with transition temperatures above about 20 Kelvins, the possibility of using them to obtain greater efficiency in electrical and magnetic applications has attracted considerable interest.
Although superconducting oxides have complex, brittle, ceramic-like structures which cannot by themselves be drawn into wires or similar conductor forms using conventional metal-processing methods, progress has been made in manufacturing superconducting oxide conductors with high engineering current capacity as composite structures in which the superconducting oxides are supported by a matrix material, typically a noble metal, which adds mechanical robustness to the composite. In preferred manufacturing processes such as the "PIT" process, in both its oxide precursor and metallic precursor variations, and coated conductors, the desired superconducting oxide is formed within or upon the supporting matrix by a combination of phase transformation and oxidation reactions which occur during the manufacturing process. For example, the well-known powder-in-tube ("PIT") process, which is used to prepare composites in elongated forms such as wires, tapes and cables, includes the steps of: (a) forming a powder of superconductor precursor material; (b) filling a metal container, such as a tube, billet, or grooved sheet, with precursor powder and deformation processing one or more filled containers to provide a composite of reduced cross-section including one or more filaments of superconductor precursor material in a surrounding metal matrix; and (c) further thermomechanically processing the composite to form and sinter filament material having the desired superconducting properties. Multifilamentary composites with the desired number of filaments may be obtained by successive rebundling or cabling iterations, with additional deformation and thermomechanical processing steps as needed.
A key requirement for improving the current-carrying capacity (J.sub.c) of polycrystalline superconducting ceramics is a high degree of densification and crystallographic alignment or texture of the superconducting grains. In conventional PIT processing, an initial deformation stage, commonly called the breakdown stage, is used to reduce a large diameter, low density precursor composite to a highly aspected, high density tape, cable or wire via one or more deformation drafts. Total reductions in excess of 40% during the breakdown stage are common. During the breakdown stage, the grains of the precursor phases are densified and physically aligned in relation to the direction of elongation, namely primarily such that the c-directions of the grains are aligned orthogonally to the desired current direction along the composite axis, which promotes good intergrain electrical connectivity after phase conversion. In fine multifilamentary composites, the breakdown stage also forms the basic shape and cross-section of the filaments in order to promote reaction induced texture during subsequent heat treatments. Flat, evenly shaped filaments in which one dimension of the filament is no greater than about the longest dimension of the desired superconducting grains have been found to provide improved J.sub.c performance. Additional intermediate deformation stages, typically at low reductions, may be used after the breakdown stage to reduce the severity of reaction induced defects in the textured superconductor phases and to modulate the mosaic spread of its grains in order to further improve its texture. Between deformation stages, reaction sintering heat treatments are used to convert the oxide particle fragments of the precursor to the desired superconductor or to an intermediate phase, typically also a superconductor, to heal cracks induced by deformation, and to promote texturing by enhancing the anisotropic growth of the superconducting grains. This type of uni-axial texturing has been particularly well developed for the PIT fabrication of the micaceous bismuth-strontium-calcium- copper-oxide (BSCCO) 2223 and 2212 superconducting phases (Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-x and Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8-x respectively), because these oxides exhibit a modest amount of plastic deformation via the activation of the a-b plane slip systems, although it has also been applied with modest success to the rare earth-containing superconducting copper oxides, the thallium-containing superconducting copper oxides and the mercury-containing superconducting copper oxides. Typical prior art processes use a breakdown stage followed by one to four intermediate stages for a total of two to five iterations, each typically involving multiple deformation drafts, although processes employing a breakdown stage with a single draft and, in one embodiment, no further iterations, have also been disclosed. See, for example, co-pending application U.S. Ser. No. 08/468,089, (US '089) filed Jun. 6, 1995 and entitled "Improved Deformation Process for Superconducting Ceramic Composite Conductors", which is herein incorporated in its entirety by reference. The deformation sequence may be designated by the term "nDS", in which "D" refers to the deformation step, "S" refers to the sintering or heating step and "n" refers to the number of iterations. Common deformation techniques in nDS processes include extrusion, drawing, roll working, or pressing. The various forms of roll working, such as strip rolling, groove rolling, rod rolling, cover rolling, and turk's heading, are particularly well-suited for continuous processing of long lengths of superconducting material, such as wire, tape or cable. Like many "nDS" techniques, the approach described in US '089 also has an extremely sensitive process response surface, with small variations from optimum processing parameters creating high dimensional variations and large decreases in J.sub.c. This creates difficulties for large scale manufacturing, as extremely precise control over deformation conditions is hard to maintain over extremely long lengths of wire, cable or tape.
However, to be practical outside the laboratory, most electrical and magnetic applications require that the conductor be manufacturable to precise dimensions at reasonable cost, in addition to having high current-carrying capacity. Long, flexible conductors with specific geometries and low dimensional variations are needed in many applications, including coils, rotating machinery and long length cables. In comparison to conventional conductors of comparable cross-section, high performance superconducting ceramic composites are quite difficult to manufacture to precise specifications and tolerances because of the complexity of the manufacturing process, the number of in situ chemical reactions involved, and the difficulty in predicting the interactions between the components of the composite during the various deformation stages.
Deformation processing of any material is complex. Standard metal-working processes have both imposed stress and imposed displacement boundary conditions. In roll working, for example, standard process parameters that control these conditions include front and back tension, roll diameter, reduction, and friction coefficient. See, e.g. Avitzur, "Handbook of Metal-Forming Processes", Ch. 13 for a discussion of the non-linear interactions of process parameters for roll working in the simplest case, that of deformation of a pure metal in a system where lateral spread of the metal cannot take place. The influence of some process parameters, such as roll diameter and friction, is not easily predicted. However, for pure metals, certain parameters, such as tension, will most directly influence stress and others, such as reduction, will most directly influence strain. Thus, high front and back tensions are conventionally used in the manufacture of metal wires to improve control of wire dimensions. Up to the yield strength of the metal, higher tensions are associated with lower dimensional variations in the wire.
The situation is more complex in a composite material for which there are discontinuities in materials parameters at each internal interface between one material and another. Where there are significant differences in mechanical properties, such as hardness, between the two materials, as there are between superconducting ceramics and typical matrix materials, the composite architecture can be very important in determining the dominant effects. Moreover, the material properties and processing parameters can interact in unpredictable ways. For example, deformation of a precursor powder may increase its structural integrity over time due to compaction, or decrease it due to breakup of the powder grains and/or macroscopic shear failure.
For superconducting ceramics, the constraints of the manufacturing process limit the ranges over which many process and product parameters may be varied if a composite with high J.sub.c is to be obtained. For example, the composite architecture and starting dimensions are constrained by the need for uniform heating and oxygenation of the ceramic filaments throughout the composite cross-section. Without this uniformity, poor precursor conversion reactions result in composites with lower J.sub.c s. High reductions, particularly during the breakdown pass, have been associated with improved texturing and filament density, but excessive reductions have been associated with cracking (cf '089 and co-pending application U.S. Ser. No. 08/651,688 filed of even date and entitled "Improved Breakdown Process for Superconducting Ceramic Composite Conductors", which is herein incorporated in its entirety by reference), so the acceptable range of reduction factors may be quite narrow. Moreover, as mentioned above, optimum J.sub.c s are obtained when the filament cross- section in the finished composite falls within certain narrow bounds determined by the grain size of the superconducting material. Delicate balancing of the composite geometry and reduction factor is needed just to achieve the desired filament cross-section, without regard to precise control of other composite dimensions. Roll diameter and friction have been identified as important factors in obtaining suitable core density and avoiding composite defects, so appropriate choices for these parameters may also be quite limited.
Low dimensional variations are a key product specification for many applications of superconducting ceramic composites, such as coils and cables. In conventional wire processing, high front and back tensions are known to produce low dimensional variation, but in superconducting ceramic composites, excessive tension has been shown to lead to strain localization which causes poor filament uniformity and reduced J.sub.c.
Therefore, it is an object of this invention to provide a method for forming a high performance superconducting ceramic composite article having low dimensional variation, good filament uniformity, and high J.sub.c over long lengths.
It is a further object of this invention to provide a method for controlling the dimensional variation of a multifilamentary superconducting article which is effective despite variations in processing conditions typical of large-scale manufacturing operations, and which may be used to optimize composite uniformity at reasonable cost and in a limited number of processing steps.